allow service to have nil handler

@perfmode this means we can create a Service first, give it to
the network and protocol (removing interdep).
bugfix: service has a Start func

We were issuing handling goroutines in both NewService and Start
Service: remove RequestID from SendMessage
Move Sender interface to network pkg

@perfmode sender is exactly what we need to pass in to dht/bitswap.
better protobuf Makefile with wildcard.